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What is an Oxygen Sensor?

So, you come in to Alex’s Autohaus to get a check engine light diagnosed and Willy tells you that it is an oxygen sensor. So, what the heck is an oxygen sensor and what does it do?

An oxygen sensor is an electronic device that helps modern fuel injected engines control the air to fuel ratio of your engine. The ideal air/fuel ratio is called the stoichiometric ratio. This varies by the type of fuel, but the most common ratio is 14.7:1 for gasoline engines. A ratio lower than this indicates a lean fuel mixture (not enough fuel), and a higher ratio is a rich fuel mixture. The oxygen sensor corrects this depending on the type of sensor.

Oxygen sensors can broadly be defined as being either wide-band or narrow-band.

A narrow band sensor is calibrated to know three things, rich, lean and stoichiometric. As the car is running the sensor reads the oxygen concentration compared to atmospheric and sends a signal back to the vehicles computer. If the mixture reads rich the computer reduces the amount of fuel, and if lean it adds fuel. A narrow band oxygen sensor essentially acts like a switch to the car to get the correct air to fuel ratio. The sensor doesn’t know the exact ratio only which side of stoichiometric that it is.

A wide band sensor is a much more sophisticated sensor that can tell you the exact air/fuel ratio of the car. This is helpful in improving emissions, fuel economy and performance. The idea is still to get to stoichiometric , but a wide band does it much more precisely. Instead of an on/off switch moving rapidly back and forth think more of a dimmer switch that refines to exactly what the car needs.

There are many more components to the system, but this gives you a broad look at what these sensors do. As you can see, a bad oxygen sensor can result in poor fuel economy due to too much fuel and decreased performance. I have seen estimates that a bad oxygen sensor can impact fuel economy by up to 40%! A bad sensor can also cause other damage like ruining a catalytic converter (not cheap). So next time you overhear someone say that they have a bad oxygen sensor, but that they’re not too worried about it you will know better.